Electric discharge machining (EDM) apparatus

ABSTRACT

The EDM apparatus for machining a workpiece by electric discharge while supplying dielectric fluid which contains powder to the work gap formed by a tool electrode and the workpiece to be machined includes a cover to protect them against the adherence of the powder that is contained in the dielectric fluid, a fluid feeding device supplying the powder-containing fluid to the work gap during the electric discharge machine process, and, at the same time, also supplying fluid that does not contain any powder to the parts inside the cover.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Phase Application of PCT/JP94/00979 filed 16 Jun.1994 and based, in turn, on Japanese National Application 5-181825 filed16 Jun. 1993 under the International Convention.

FIELD OF THE INVENTION

The invention relates to an EDM apparatus, i.e. a machine which uses adielectric fluid containing powder or a dielectric fluid containing nopowder as the machining medium.

BACKGROUND OF THE INVENTION

Generally, EDM apparatus can be divided into two types, die-sinking EDMmachines and wire-cut EDM machines. A die-sinking EDM machine uses a3-dimensional tool electrode with a dielectric fluid, such as, forexample, an aqueous solution containing kerosene or a high molecularweight compound, in the gap between the electrode and the workpiece, asthe machining medium. It machines the workpiece by applying voltage tothat gap and generating an electrical discharge across the gap.

On the other hand, a wire-cut EDM machine uses wire made from suchmaterial as brass or tungsten of a diameter of 0.02 to 0.03 mm as thetool electrode. It machines the workpiece by running the wire injuxtaposition with the workpiece between a pair of wire guides. Itgenerally uses deionized water or a kerosene based fluid as thedielectric fluid that is present between the wire and the workpiece.

In finish-machining, it is known that using a dielectric fluid with, forexample, finely powdered polycrystalline silicon, mixed in it, willprovide a good surface roughness on the workpiece and will improvemachining stability. British Patent No. 828,336 and U.S. Pat. Nos.4,392,042 and 4,952,768 disclose the use of this sort of fluid.

If this fluid is used in a wire-cut EDM machine, it could affect thetravel of the wire by, for example, powder entering the rotating partsin the work tank, such as the bearings in the rollers provided along thewire conveying route, and, as a result, causing an undesirable effect onthe machining. Furthermore, if the powder enters the movable wire guidesor into the wire guide assemblies containing the movable electricityfeeding elements which supply electricity to the wire, it could hinderthe movements of those guides and elements. Also, much labor is requiredto remove the powder that remains in the wire conveying mechanism andguide assemblies when the machining work is finished.

OBJECTS OF THE INVENTION

one of the objects of the present invention is to provide an EDMapparatus in which powder-containing fluid does not adhere to thenecessary parts in the work tank.

Another object of the present invention is to provide an EDM apparatusin which the work of removing the powder-containing fluid adhering tothe necessary parts in the work tank is reduced.

SUMMARY OF THE INVENTION

In order to achieve the foregoing objects, the EDM apparatus of thepresent invention, is an apparatus which uses tool electrodes located inthe work tank and machines the workpiece by electrical discharge. It hasa housing that accommodates such sliding parts as the electricityfeeding element and wire guides for the wire electrode, and has a fluidfeeding device that, while supplying powder-containing dielectric fluidto the working gap formed by the wire electrode and the workpiece,introduces powder-free fluid into that housing.

Also, desirably, the EDM apparatus should contain a cover which housesthe movable parts located along the wire conveying route, and the fluidfeeding device should, during the EDM process using powder-containingfluid, introduce powder-free fluid into that cover.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and new features of the presentinvention will become more readily apparent from the followingdescription, references are being made to the accompanying drawing inwhich:

FIG. 1 is a schematic drawing illustrating an embodiment of the EDMapparatus of the present invention;

FIG. 2 is a sectional view of an embodiment of the upper guide assemblyshown in FIG. 1;

FIG. 3 is a sectional view of another embodiment of the upper guideassembly;

FIG. 4 is a schematic drawing illustrating the wire take-up device;

FIG. 5 is a sectional view of the wire take-up device shown in FIG. 4taken along the line V--V of FIG. 4;

FIG. 6 is a sectional view of another embodiment of the wire take-updevice; and

FIG. 7 is a sectional view of yet another embodiment of the wire take-updevice.

SPECIFIC DESCRIPTION

FIG. 1 shows an embodiment of a wire-cut EDM apparatus. In thatapparatus, the wire electrode 1 is continuously fed from a wire feeding(not shown in the drawing), and is guided by a pair of wire guides,described later, to the gap formed by wire 1 and the workpiece 2 locatedin the work tank 5. In the embodiment illustrated, the pair of wireguides is located respectively above and below the workpiece 2 and ispart of the guide assemblies 3 and 4. During the EDM process, voltagepulses are supplied to the gap intermittently by a power supply device(not shown in the drawing), to generate electric discharges and machinethe workpiece 2.

In the embodiment illustrated, a dielectric fluid feeding device 6 isprovided which selectively feeds to the work gap oil-based dielectricliquid 7 containing no powder and oil-based dielectric liquid 8containing powder. The fluid feeding device 6 consists of tank 20 forholding fluid 7 containing no powder, tank 30 for holding used fluid 7discharged from work tank 5, and tank 40 for holding fluid 8 containingpowder. Desirably, the dielectric fluid 7 (free from powder) should beused for roughing and the fluid 8 (with the powder) for finishing.

The feed route of inductive fluid 7 is explained herewith by referringto FIG. 1. By operating pump 21 when preparing to machine, fluid 7 intank 20 is fed, via solenoid valves 22 and 23, from feed inlet 5A intotank 5 and fills work tank 5 with fluid 7. During the machining process,by operating pump 21, the fluid 7 in tank 20 is injected from therespective nozzles of guide assemblies 3 and 4, via solenoid valves 22and 24, into the work gap. In a wire-cut EDM machine, desirably, anautomatic wire threading device should be provided so that, at the startof machining or when the wire breaks, wire 1 is automatically threadedthrough the work gap of workpiece 2 and strung between the pair of wireguides. For example, in Japanese Official Unexamined Patent PublicationNo. 62-13029, an automatic wire threading device is disclosed wherebythe wire is threaded between the work gap and the lower wire guide byintroducing the wire into a pipe that can be raised or lowered and usingthe jet flow created by feeding pressurized fluid into the pipe, tothread the wire. IN the present embodiment, by operating pump 21, fluid7 in tank 20 is fed, via solenoid valves 22 and 27, to the automaticwire threading device to provide the necessary jet flow. In work tank 5,an overflow outlet 5B is provided to discharge overflow fluid andmaintain the fluid of work tank 5 at a fixed level. The dischargedoverflow fluid is sent, via solenoid valve 29, to tank 30. Whenmachining is finished, dielectric fluid 7 in work tank 5 is sent, viasolenoid valves 28 and 29, to tank 30. By running pump 31, fluid 7 intank 30 is returned to tank 20 via filter 32 which removes the chipsthat are produced during the electric discharge machining.

Next, the feed route of dielectric liquid 8 (with the powder) isexplained herewith. By operating pump 33 when preparing to machine,dielectric fluid 8 in tank 40 is fed, via solenoid valves 34 and 23,from feed inlet 5A into work tank 5 and fills work tank 5 the dielectricliquid. During the machining process, by closing valve 25 and operatingpump 33, the dielectric liquid 8 in tank 30 is injected from therespective nozzles of guide assemblies 3 and 4, via solenoid valves 34and 24, into the work gap.

In addition, fluid not containing powder 1 is fed into the guideassemblies so that powder does not enter the part housing the movableparts such as, for example, the wire guides and electricity feedingelement. The fluid 7 may be used to prevent the ingress of powder.Desirably, fluid made free of powder by filtering fluid 8 should beused. In the embodiment illustrated, backwashing filter 37 is providedin order to remove the powder from fluid 8. By closing valve 25 andoperating pump 35, the fluid 8 in tank 30 is fed to guide assemblies 3and 4, via solenoid valve 36, backwashing filter 37, and solenoid valves38 and 39. In addition, in order that powder does not enter the rotatingparts provided in the work tank, fluid 8 is fed to the wire conveyingroute via backwashing filter 37 and solenoid valves 38 and 41. Also, itis desirable that during the automatic wire threading process, byoperating pump 35, fluid 8 be fed to the automatic wire threading devicevia solenoid valve 36, backwashing filter 37 and solenoid valves 38 and27. Overflow fluid is sent from overflow outlet 5B to tank 40 viasolenoid valve 42. When machining is finished, fluid 8 in work tank 5 issent, via solenoid valves 28 and 42, to tank 40.

As the dielectric fluid 8 with powder is used only for finishing, thequantity of chips produced by electric discharge machining is relativelysmall. In order to remove those chips, a filtering device such as acentrifugal filter or a magnet filter may be provided. In addition, adensity controlling device, including a density detector, may beprovided to control the concentration of the powder in the dielectricliquid 8. An agitator may also be furnished to prevent the settling ofpowder.

When the machining process is over, dielectric liquid 8 will remain atthe bottom of work tank 8 and in guide assemblies 3 and 4, or willadhere to the work stand and jig that support workpiece 2. After thefluid has evaporated, the powder adheres firmly to the components inwork 5 and is difficult to remove. When cleaning this work tank 5, it isdesirable to wash off the powder-containing residual fluid by operatingpump 35 and introducing fluid 8 into work tank 5 via solenoid valve 36,backwashing filter 37, solenoid valves 43 and 23 and feed inlet 5A. Inaddition, when necessary, fluid 8 may be fed, via solenoid valve 36,backwashing filter 37 and solenoid valves 38 and 39, to the respectivenozzles of guide assemblies 3 and 4 to remove any powder remaining inthose nozzles. Fluid 8 used fro the above cleaning process is dischargedfrom work tank 5 and returned to tank 40 via solenoid valves 28 and 42.

In order to return to tank 40 the powder that was caught by backwashingfilter 37, fluid 8 in tank 40 is circulated via solenoid valve 44,backwashing filter 37 and solenoid valve 45, by closing valves 36, 38and 43, and operating pump 35 for a fixed period, to recover the powderthat was caught on backwashing filter 37.

As described in the foregoing, in the present embodiment, by sending,not fluid 7, but fluid 8 to work tank 5 via the backwashing filter,ingress of powder into the movable and sliding parts is prevented or thepowder remaining in work tank 5 after machining is done using fluid 8,is removed. Therefore, the concentration of powder in tank 40 ismaintained at a uniform level after the powder that has been caught onbackwashing filter 37 is recovered, and it is easy to exercise controlover the density of the powder. Also, fluid 8 sent from tank 40 and thepowder remaining in work tank 5 are always returned to tank 40, so thereis no loss of precious powder.

Note that in the present embodiment of the wire-cut EDM machine, controlunit 9 is furnished to control the solenoid valves and pumps shown inFIG. 1. This control unit responds, for example, to signal S whichindicates the operating state of the machine proper, decides on theoperating sequence of the solenoid valves and pumps employing amicrocomputer, and outputs control signal CS. Or, control unit 9 may bemade to output control signal CS by reading commands input manually byan operator or commands read out from a program.

Upper guide assembly 3 shown in FIG. 1 is explained herewith byreferring to FIG. 2. Upper guide assembly 3 includes nozzle housing 56.This housing 56 forms not only chamber 57 but also includes nozzle 70opposite workpiece 2. Also, there is a hole made in housing 56 in orderto thread wire 1. This hole 59 is of a diameter that permits a pipe (notshown in the drawing) to pass through it in the automatic wire threadingprocess. During the machining process, dielectric liquid is introducedfrom feed inlet 90 into chamber 57 and injected toward the work gap fromnozzle 70.

Guide assembly 3, additionally includes guide housing 54 which isfurnished on top of the nozzle housing. This guide housing 54 formschamber 55. In this chamber 55 is housed electricity feeding element 60which feeds electric current from the power supply to wire 1 through asuitable feeder line. This electricity feeding element is attached to amovable part 64 activated by actuator 65 so that the element can eitherpress against or move away from wire 1. In addition, in chamber 55,relative to electricity feeding element 60, is housed wire guide 51located on the top and wire guides 52 and 53 located on the bottom.These wire guides each have a V-shaped groove in them. Upper wire guide51 is attached to a movable part 63 activated by actuator 58 so that itcan either contact or move away from wire 1. Lower wire guides 52 and 53are respectively attached to movable parts 63 and 64. During themachining process, guides 52 and 53 are made to come into contact byactuators 58 and 65 so that they form a guide hole, only slightly largerthan the diameter of wire 1, leading to hole 59. This guide holedetermines the position of wire 1 relative to workpiece 2. When wire isbeing automatically threaded, electricity feeding element 60 and guides51, 52 and 53 are moved away from wire 1 by actuators 58 and 65.

So that powder-containing fluid does not enter chamber 55 from die guide69 attached to guide housing 54 and then from hole 59, fluid notcontaining powder is introduced from feed inlet 80. This causes wire 1to be accurately positioned by the smooth movement of movable parts 63and 64 and the contacting of guide 52 and 53. Also, cleaning aftermachining is not necessary since powder-containing fluid will not remainin such a confined space as the V-shaped groove of the wire guides. Notethat the pressure of the fluid that is introduced from feed inlet 80should, desirably, be controlled to the same level as or slightly higherthan the pressure of the fluid introduced from feed inlet 90.

Another embodiment of the upper guide assembly is explained herewith byreferring to FIG. 3. Upper guide assembly 3A includes nozzle housing 61.This housing 61 forms not only chamber 62 but also includes nozzle 70opposite workpiece 2. During the machining process, fluid is introducedfrom feed inlet 90 into chamber 62 and injected toward the work gap fromnozzle 70.

Guide assembly 3A, additionally includes guide housing 66 which isfurnished on the inside of the nozzle housing 61. This guide housing 66forms chamber 67 and also houses movable part 71. This movable part 71can be moved by actuator 72 in a direction perpendicular to the axis ofthe travelling wire. This movable part 71 has a hole 73 made in it toallow wire 1 to be threaded through it. Flat-shaped electricity feedingelement 60 is attached to this movable part 71 by means of a set screw74 so that it can either contact or move away from wire 1.

Since this electricity feeding element 60, by loosening set screw 74,changes its location of contact with wire 1, it is perpendicular to theaxis of travelling wire 1 and can slide perpendicularly to the directionof travel of movable part 71.

In addition, wire guide 50, which is near nozzle 70 and determines theposition of wire 1 opposite workpiece 2, is formed in guide housing 66.From this wire guide 50 and from die guide 69 attached to guide housing66, fluid not containing any powder is introduced from feed inlet 80 sothat fluid containing powder does not enter chamber 67. This serves tomake movable part 71 move smoothly and electricity feeding element 60 toslide smoothly. Also, since fluid containing powder does not remain insuch a confined part as wire guide 50, there is no need to do anycleaning after machining. As stated before, the pressure of the fluidthat is introduced from feed inlet 80 should, desirably, be controlledto the same level as or slightly higher than the pressure of the fluidintroduced from feed inlet 90.

The present invention can also be applied to the case of movable partsbeing used in the wire take-up mechanism, located in the work tank,which carries the used wire to its prescribed place of discharge.

Wire take-up device 100, which is supported by lower arm 120 and whichcarries, to the prescribed place of discharge, used wire 1 which is sentout from lower guide assembly 4, is explained herewith by referring toFIG. 4 and 5.

Wire take-up device 100 contains belts 101 and 102 which convey the wireand a number of rollers 103 which make those belts move. These rollers103 are attached to arm 120 which supports lower guide assembly 4. Inaddition, case 116, which encloses those belts and rollers 103, is fixedto arm 120 by means of bolts 113 and 114.

As best depicted in FIG. 5, roller 103 contains roller shaft 115, innerring 104 and outer ring 195. Roller shaft 105 is fixed to arm 120 bymeans of bolt 111. Inner ring 104 is furnished on the outside of rollershaft 115 via key 123. Outer ring 105 is furnished on the outside of 104via a number of rolling elements.

In addition, cover 122 is attached to roller shaft 115 by means of bolt110 and also to case 116 by means of bolt 112. Cover 122 is providedwith feed inlet 119. The fluid that, as stated before, is fed from tank40 via solenoid valve 36, backwashing filter 37 and solenoid valves 38and 41, is made to flow from feed inlet 119 into void 124 formed by thecase and cover 122. A very small amount of the fluid flowing into void124 flows out of flow outlet 121 and guide hole 117 formed in cover 116,and is regulated so that powder-containing fluid does not ingress intovoid 124. Therefore, in particular, powder does not adhere to therolling elements used in the rollers.

FIG. 6 is a section view of another embodiment of the wire take-updevice. The parts that are the same as the parts in FIG. 5 bear the samenumbers and so their explanations will not be repeated.

In the embodiment illustrated, pulley 118 is furnished on the outside ofouter ring 105 via key 123. This pulley 118 has a V groove to guidewire 1. Cover 125 enclosing the rollers 103 is attached to arm 120 and,at the same time, to lower guide assembly 4. The fluid that is fed viavalve 41 passes through an appropriate feed pipe inside arm 120 and isintroduced via feed inlet 119 to void 124 formed by cover 125.

FIG. 7 is a section view of yet another embodiment of the wire take-updevice.

In the embodiment illustrated, cover 126 is attached to arm 120 andencloses rollers 103 individually.

The present invention is not for the purpose of limiting its form to onethat is exactly identical to the disclosed form because it is clear fromthe above explanations that there are many possible improvements andvariations. The above embodiments were selected to explain the essenceand the practical application of the invention. The scope of theinvention is defined by the scope in the attached application forpatent.

I claim:
 1. An EDM machine comprising:a work tank; a tool electrodemounted in said work tank; means for mounting a workpiece to be machinedin said work tank for juxtaposition with said tool electrode, saidworkpiece to be machined and said electrode forming a gap therebetweenin said work tank; means for storing a first fluid containing a powderysubstance; means for storing a second fluid free from said powderysubstance, cover means bounding a space containing parts to be protectedfrom said first fluid; and fluid feeding means for selectivelydelivering said first fluid to said gap and said second fluid to saidspace whereby said sectional fluid excludes said first fluid from saidparts and an electric discharge across said gap machine said workpiecewith said tool electrode.
 2. The EDM machine defined in claim 5 whereinsaid parts include an electricity feeding element enclosed in said covermeans and operatively connected with said tool electrode, said toolelectrode being a travelling wire forming said gap with said workpieceto be machined.
 3. The EDM machine defined in claim 2 wherein said covermeans comprises a housing, and displaceable guide means in said housingdefining a feed path of said travelling wire for determining position ofsaid tool electrode with respect to said workpiece.